1. Field of the Invention
The present invention relates to a power steering device for use in motor vehicles in general and more particularly to a power steering device which undergoes a speed sensory control and a responsiveness control.
2. Description of the Related Art
A popular power steering device for use in motor vehicles is a speed sensing type. However, a motor vehicle provided with a power steering device which undergoes a responsiveness control is now demanded in view of the desired drivability and stability.
Further, in order to enhance drivability and stability of motor vehicles, it is necessary to investigate performance from two aspects because motor vehicle performance changes in response to steering operations of drivers, and in response to disturbances such as road irregularity and gusts of wind.
For the former aspect, one must consider increased motor vehicle speeding and the advanced ages of motor vehicle drivers which causes an expansion of in the range of physiological ability levels in relation to driving skill. It is therefore necessary to alleviate problems caused by the increased speeding and the expansion in the range of physiological ability levels within the motor vehicle side, i.e., the former aspect.
For example, an expressway in the middle of the night is as if it were a huge conveyer belt filled with trucks. These trucks are driving towards the metropolitan area. The distance between these truck groups is generally short and the speed thereof is high. Further, highway buses between cities aer present. Because of their advantages such as inexpensive fairs and ease of use, living space outside the city with a high quality, and attentive services, not only are day time highway bus services increasing, but also night time bus services are increasing. Night bus services were understood at the beginning to be used only to supplement railway train services. This is no longer true. As such, on one hand, high speed and long distance services are active. However on the other hand, in particular, with regard to trucks, the shortage of truck drivers is serious, and in addition, the age of the drivers has been increasing. Further, there appears to be an indication of woman driver expansion. The higher the speed of the motor vehicle is, the more the response performance thereof reduces. Still further, physiological abilities relating to driving skill of advanced age drivers and woman drivers are relatively low in comparison with those of young man (see FIG. 1). A system is desired which realizes a compensation in the motor vehicle for the decrease of response performance at high speeds and the physiological ability difference of the drivers. Moreover, a large sized motor vehicle, which requires a relatively wide space on the running road in comparison with a passenger car, should be provided with a better response performance than the passenger car.
Further, with regard to the desirable motor vehicle response characteristics, there are reports, one of which defines T.beta., a factor corresponding to the product of a time constant and yaw gain. The report indicates that the smaller the products is, the higher the subjective judgement of the driver is (see FIGS. 2a through 2c). Another report indicates that there is an optimum region of moving direction holding property in a range of small time constants and of certain amounts of yaw acceleration again (see FIG. 3). These data are concerned with passenger cars. However, these tendencies are similar in trucks and buses. FIG. 4 shows an example of the response characteristic of a truck and a bus, wherein the truck is equipped with a front engine and a leaf suspension and the bus with a rear engine and an air suspension. The gain and the phase lag of the truck are smaller than those of the bus. Further, the subjective judgement of the truck driver is better than the bus driver. The gain and phase lag increase in response to an increase in the vehicle speed (see FIG. 5) and thereby burdens to drivers increase correspondingly. In view of FIG. 4 and FIG. 5, a desirable steering response is aimed at reducing both the gain and the phase lag to less than they are now. This tendency meets with the previous tendency with regard to the passenger car data, when the phase lag is assumed to belong to the time constant. When the phase lag is large, the approach to a course aimed at by a driver lags so that excess turning or swing is caused in an effort to compensate. When the gain is much larger, the excess turning is amplified so drivers can be compensated for by reducing the time constant in that the motor vehicle becomes unsteady. Still further, the problems arising from the physiological ability differences of the steering system.
Although the above description deals with the response performance with regard to the driver's steering force, the same is true with regard to road disturbances. For example, because of a disturbance 1 due to a road irregularity, an axle pedal displaces 2 and then the suspension 3 and vehicle body displace 4 which displacements are sensed 5 by the driver. When an adjusting operation is carried out, any lag included in the processes 1-5 of the adjusting steering operation causes the deviation from the course to become large accordingly. The displacement in this case (more precisely the displacement into the direction which causes the deviation from the course) corresponds to the gain and is self-evidently preferable to be minimized. Namely, such disturbance should be prevented before they take effect at the entrance.
Further, when the steering response performances of trucks and buses are compared with those of passenger cars, there is a large difference in the phase lag, which is very large in trucks and buses (see FIG. 7). For analyzing what causes make the phase lag to be so large, the time lag from the initiation of the steering operation to the initiation of a change in the moving direction was measured along its transfer route (see FIGS. 8a through 8f). The time lag in the steering system occupies 60% of the total lag (see FIG. 9). The remaining 40% is the time lag in the vehicle body system, and the time lag due to the fact that buses are larger than trucks, the ultimate reasons for the lag are the influences of their suspension structure and differences between load allotments to front and rear shafts. Trucks and buses employ a common steering device so that with regard to the time lag in the steering device, there is no difference.
Further, several studies were carried out to shorten the time lag in the steering system which occupies 60% of the total lag. Some of these went further to suggest specific system structures but failed to reach practical uses.
Two reasons are presumed for the failures. One is that although the phase lag has been shortened, concurrently the gain gets large thereby the steering wheel becomes too sensitive and the steering feeling is deteriorated. The other reason may be that since the mechanical coupling between the steering wheel and the front wheels has been disconnected, problems with safety are prevalent.